1. Field of the Invention
The present invention relates to a method of manufacturing an optical element having an optical surface of a predetermined shape. In particular, the manufactured optical element has an optical surface of a predetermined shape extending close to a periphery of a substrate on which the optical surface of the predetermined shape is formed. In particular the invention also relates to the manufacture of an optical element having an optical surface of an aspherical shape.
2. Brief Description of Related Art
An optical element having an optical surface can be, for example, an optical component such as an optical mirror or an optical lens, used in an optical system, such as a telescope used in astronomy, or an optical system used for imaging structures of a mask, such as a reticle, onto a radiation sensitive substrate, such as a resist, in a lithographic process. The success of such an optical system is substantially determined by the accuracy with which the optical surface of its components can be machined or manufactured to have a designed target shape.
A conventional method of manufacturing an optical element comprises processing of the optical surface such that differences between a shape of the optical surface and a target shape thereof are within given tolerances. The tolerances depend on the application for which the optical surface is designed and can be chosen by one of ordinary skill in the art based upon a desired application. Typically, tolerances are lower for applications using shorter wavelengths of the light used in the imaging application. Further, different tolerances can be defined for different spatial length scales over which surface variations occur. Such spatial length scales are also referred to as spatial wavelengths or spatial wavelength ranges. For example, different tolerances can be defined as rms values of a distribution of the differences between the surface shape and its target shape in a lateral direction of the surface for different spatial wavelengths. For example, a first tolerance is defined for differences in a low spatial wavelength range (LSFR) in the order of about millimeters to some 10 centimeters, which corresponds to dimensions in the order of about one tenth of a diameter of the optical surface up to the diameter of the optical surface. Such tolerances typically represent a shape error of the optical surface. Shape errors of the optical surface contribute to aberrations of an optical system in which the optical surface is used.
Processing for reducing deviations in the low spatial wavelength range typically include milling, grinding, fine grinding, such as loose abrasive grinding, polishing and others. Methods of measuring deviations or surface errors of this spatial wavelength range typically include interferometric measuring methods using measuring beams of light having a diameter corresponding to the diameter of the optical surface or less, if a stitching method is used in which measuring results of portions of the optical surface are stitched together to achieve a measuring result indicative for the surface shape of the whole surface.
Polishing tools which are in contact with the optical surface at comparatively large contact surfaces are typically used to reduce deviations in a medium spatial wavelength range (MSFR) of the order of about millimeters down to about micrometers, and in a high spatial wavelength range (HSFR) in the order of about micrometers down to about the wavelength of the light used in the application. Tolerances defined for the medium spatial wavelength range and the high spatial wavelength range typically represent a surface roughness of the optical surface. Surface deviations in the medium spatial wavelength range may contribute to a stray light or flare within an optical field of the optical system in which the optical surface is used. Surface deviations in the high spatial wavelength range may contribute to a reduction of reflectivity of the optical surface if used as a mirror in an optical system.
For polishing an optical surface in a region of a periphery thereof, it is necessary that the main surface of the substrate on which the optical surface is formed extends beyond the periphery of the optical surface such that the polishing tool can reciprocate in the region of the periphery of the optical surface while maintaining a steady and continuous pressure towards the optical surface. If a peripheral region of the main surface surrounding the optical surface were not provided, there would be a risk of deteriorating the global shape of the optical surface, i.e. of increasing deviations in the low spatial wavelength range and possibly also even in the mid spatial wavelength range, in a region close to the periphery of the optical surface by polishing.
The peripheral surfaces surrounding the optical surface of the optical element are often used as contact surfaces for mounting the optical element in suitable mounts. In some applications however, it is desirable that the optical surface of the predetermined shape extends close to the periphery of the substrate. An example of such an application is a mirror used in a projection optical system for imaging structures of a radiation sensitive substrate in a lithographic process using extreme ultraviolet (EUV) radiation. In this application, the optical element is a mirror disposed in a folded beam path, and a substrate of the mirror extending beyond the mirror surface would obscure portions of the beam path.